In the art of lithography, it is desirable to produce a multicolor proof to assist a printer in correcting a set of color separations which will be used in exposing a series of expensive lithographic printing plates. The proof should reproduce the color quality that will be obtained during the printing process. The proof must be a consistent duplicate of the desired half tone or line image. Visual examination of a color proof should reveal the color rendition to be expected from a press using the color separations, and any defects on the separations which might need to be altered before making the printing plates. Color proofing sheets for multicolored printing can be made by using a printing press or proof press. This requires that all of the actual printing steps be formed. However, this conventional method of color proofing is costly and time consuming. Alternate color proofing methods have therefore been developed to simulate the quality of press proofs. There are two general types of photoimaging methods, namely the overlay type and the single sheet type
In the overlay color proofing method, an independent transparent plastic support is used for producing a colored image of each color separation film. A number of such supports carrying the various colored images of cyan, yellow, magenta and black are then superimposed upon each other and placed on a white sheet to produce a color proof. Examples of this approach are described in U.S. Pat. Nos. 3,136,637; 3,211,553; and 3,326,682. The primary advantage of this method is that it is quick and can serve as a progressive roof by combining any two or more colors in register. However, this color proofing method has the disadvantage that the superimposed plastic supports tend to darken the color proofing sheet, and, as a result, the impression of the color proofing sheet thus prepared becomes vastly different from copies actually obtained by conventional printing.
In the single sheet color proofing method, a color proofing sheet is prepared by successively producing images of the colors from different color separation films onto a single receiver sheet. This is done by using a single opaque support and by applying toners, photosensitive solutions, or coatings of photosensitive materials of corresponding colors to the opaque support in succession. Examples of this approach are described in U.S. Pat. Nos. 3,671,236; 4,260,673; 4,366,223; 4,650,738; 4,656,114; and 4,659,642. An advantage of the single sheet type of color proof is that the color is not influenced by superimposed plastic supports. This method more closely resembles actual printing and eliminates the color distortion inherent in the overlay system.
Various processes for producing copies of an image embodying photopolymerization and thermal transfer techniques are known as shown in U.S. Pat. Nos. 3,060,023; 3,060,024; 3,060,025; 3,481,736; and 3,607,264. In these processes, a photopolymerizable layer coated on a suitable support is imagewise exposed through a color separation. The surface of the exposed layer is then pressed into contact with the image receptive surface of a separate element and at least one of the elements is heated to a temperature above the transfer temperature of the unexposed portions of the layer. The two elements are then separated, whereby the thermally transferrable, unexposed, image areas of the composite transfer to the image receptive element. If the element is not precolored, the tacky unexposed image may now be selectively colored with a desired toner. The colored matter preferentially adheres to the clear unpolymerized material.
In U.S. Pat. No. 3,721,557, a method of transferring colored images is described which provides a stripping layer coated between the photosensitive element and the support. When the photosensitive layer is exposed to actinic light and developed, the more soluble portions are selectively removed to produce a visible image. The image-carrying support is pressed against a suitable adhesive coated receptor member and, subsequently, the carrier support sheet is stripped to accomplish the transfer of the image. A fresh layer of adhesive is applied to the receptor for each subsequent transfer. U.S. Pat. No. 4,596,757 provides a method for transferring images or solid colors which are subsequently imaged. The photosensitive material comprises a carrier support having sequentially disposed thereon a release layer; a colored, photopolymerizable layer; and an adhesive layer. The material can undergo exposure, lamination to a temporary support, wet development, and then lamination to a receptor sheet. Alternatively, the photosensitive material can be laminated to a receptor sheet, undergo exposure, and then wet processed. Both processes require development in an aqueous medium. The present invention improves upon the aforesaid system whereby the image is produced by a peel apart development process thus eliminating the need for a wet development.
Peel apart color proofing systems are themselves also known as such. In prior peel apart methods such as described in U.S. Pat. No. 4,489,154, a photosensitive layer is disposed between two self supporting sheets which are typically polyester. Upon exposure, there is an imagewise differential in adhesion of the exposed and unexposed portions of the photosensitive image to one of the two sheets forming the photosensitive element. This differential is critical to the functioning of the system. Therefore, it is necessary to control the adhesive forces between the sheets and the photosensitive layers. Hence, the sheet materials on which the image is produced has been limited to those specially produced by the manufacturer of the color proofing film. This means that images can usually only be produced on special substrates provided by the manufacturer. However, in use, the printer typically wishes to see the proof on the very particular paper stock on which he eventually will print. This has not been possible with prior peel apart development methods since variant paper stocks usually do not have the surface integrity demanded by peel apart systems. The present invention improves upon this situation by modifying a desired paper stock so that it may have the necessary surface characteristics to build a peel apart image proof directly on its surface.